1. Field of the Invention
The present invention relates to an inkjet recording apparatus configured to form an image by discharging ink droplets, and, more particularly, to an inkjet recording apparatus which performs recording by discharging ink of the same color from a plurality of long recording heads and an inkjet recording method thereof.
2. Description of the Related Art
A recording apparatus is mainly utilized as an output device in a multiple electronic device or a workstation including a computer or a word processor. The output device is generally called a printer. Further, a recording apparatus is utilized as a recording unit of a copying machine. Recently, a multi function printer in which functions of a printer, a scanner, and a copying machine are integrated has been known. The multifunction printer contains a recording device as an output device which outputs data on recording paper. The output device records an image (including characters, symbols, etc.) on a recording medium such as paper, plastic thin plate, etc. based on recording information. As a recording method adopted for a recording device, a variety of methods has been proposed. For example, an inkjet type, a wire dot type, and a thermal type have been known as recording methods in which a recording head is configured to form a dot on a recording medium based on recording information. Further, a laser beam method which forms an image by irradiating a laser beam to a photosensitive drum based on recording information is also known.
Among these methods, the methods which utilize a recording head are widely used because of a compact and inexpensive structure. As one example of the recording devices which employ a recording head, a so-called serial-type recording apparatus is known which performs recording by moving a recording head in a direction (main scanning direction) intersecting with a sub scanning direction while moving a recording medium in a certain direction (sub scanning direction). The serial-type recording apparatus employs a recording head having relatively narrow recording width. The recording device of this type performs recording on a whole area of the recording medium by repeating a recording operation that scans the recording head along a main scanning direction and a conveying operation that conveys the recording medium in a sub scanning direction. Now, the recording operation of the serial-type recording apparatus is specifically described. In the recording operation, a recording head performs a scanning operation in a main direction against a recording medium which is being stopped and performs recording of an image according to the width of the recording head. When one main scan is completed, the recording medium is conveyed by a predetermined distance, and the recording head performs the next scanning operation against the recording medium which is being stopped. In this manner, by repeating the main-scanning operation to perform recording and the conveyance operation of the recording medium, the image formation on a whole area of the recording medium is performed.
Further, as another example utilizing the recording head, a full-line type recording apparatus is known. The recording head employed for the full-line type recording apparatus has an array of recording elements wider than the width of a recording medium. In the case of the above inkjet type recording apparatus, a head having an array of several thousands of nozzles which discharge ink is fixed to a main body of the apparatus, and a recording medium is conveyed in a direction intersecting with a longitudinal direction of the recording medium to perform the recording operation. In the full-line type recording apparatus, a long recording head records one line of image at one operation while conveying a recording medium sequentially to form an image on a whole area of the recording medium. In the full-line type recording apparatus which employs a recording head of the inkjet type, a recording medium passes the recording head only once to record an image. This recording method in which an image is completely formed by one recording operation is called a one-pass recording.
Among the various above-described recording methods, the inkjet type recording apparatus (inkjet recording apparatus) which performs recording by discharging ink from a recording head has an advantage in that low noise and high-speed recording are possible since non-contact recording between the recording head and recording paper is employed. The inkjet type recording apparatus further has advantages in that downsizing of recording heads is easy, hi-definition images can be formed at a high speed, and a running cost is low since recording is performed with normal paper to which no special treatment is given. Moreover, there is an advantage that by providing recording heads corresponding to a plurality of ink colors, a color-image can be formed readily.
Especially, the full-line type recording apparatus is configured to obtain a recorded image of desirable width by one recording operation, and therefore, an image forming operation can be further sped up. Also, a potential as a device for on-demand printing, for which demands have been recently increasing, is attracting attention. In the on-demand printing, it is not required that several million copies are printed like conventional news papers or magazines. The required printing speed for the on-demand printing is about a hundred thousand of sheets per hour. On the other hand, a labor saving is desired. The full-line recording apparatus is inferior to conventional printers such as an offset printer in printing speed. However, the full-line type recording apparatus has advantages that manpower can be saved because it is not necessary to make printing plates, and small batches of a variety of printings can be dealt with readily in a short time. Thus, the full-line type recording apparatus is suitable for the on-demand printing.
For the full-line type recording utilized in on-demand printing, a high resolution recording grade equal to or more than 600×600 dpi (dot per inch) is required in a case of a black-and white document that consists of sentences, for example. In a case of recording a full-colored image such as a photograph, a high resolution recording grade equal to or more than 1200×1200 dpi is required. As for printing speed of the on-demand printing, for example, in a case of an A3 size recording medium, more than 30 pages per minute is required.
Further, in the on-demand printing, an image is often recorded on several sizes of recording medium. For example, an image captured by a digital camera is recorded on a L-size medium like conventional silver halide photograph or on a small-size medium such as a post card. However, in the full-line type recording apparatus, especially in a full-line printer capable of recording a photo-quality image on large size paper, it is extremely difficult to manufacture discharge openings and inkjet recording elements all without any defect over the whole width of a recording area. For example, in order to perform recording on recording paper of A3 size in a density of 1200 dpi, about 14000 discharge openings (the recording width is about 280 mm) are required for the full-line type recording head. Accordingly, it is extremely difficult to process such a great number of ink discharge openings and their corresponding inkjet recording elements without a single defect in its manufacturing process. Even if such process is possible, there is the possibility that a non-defective rate becomes low and a manufacturing cost immensely increases.
Accordingly, it should be assumed that defective nozzles exist within a printing head mounted on the full-line type recording apparatus, and it has been proposed that a plurality of recording heads of the same color are arranged so as to complement the defective nozzles.
Japanese patent application laid-open No. 10-6488 discloses a structure configured to complement an area where recording is not performed due to a defective nozzle. More particularly, the patent document discusses a technique that complements a defective discharge nozzle on a recording head by a nozzle located on the same raster of the other recording head.
However, when the defective nozzle is complemented according to the technique disclosed in the above patent document, the nozzle that complements the defective nozzle is extremely frequently used, which accelerates a secular change and results in shortening the nozzle life.